Along with a trend for micro-fabrication of a semiconductor device, there have been suggested various techniques for reducing a width of an opening formed on a mask layer used for plasma etching. For example, as for a method for reducing (shrinking) a width of an opening 101 in an upper photoresist layer 100 formed on a wafer as a substrate shown in FIG. 10A, there is suggested a method for using a processing gas containing CH4 (methane) gas to generate a plasma because a large amount of deposits containing carbon and hydrogen can be generated from a plasma of CH4 gas. Specifically, a plasma is generated from a gaseous mixture of CH4 gas and Ar gas, and a deposit 102 generated from the plasma is deposited in the opening 101, thereby reducing a width W1 of the opening 101 (FIG. 10B) (see, e.g., Japanese Patent Application Publication No. 2007-273866 and corresponding U.S. Patent Publication No. 2007-284337).
Since, however, the deposit 102 is deposited isotropically, it is deposited on a bottom portion as well as a side portion of the opening 101 (see the deposit 102 having a thickness T1 shown in FIG. 10C). The deposit 102 deposited on the bottom portion retards etching of a target layer to be processed, e.g., a hard mask layer 103 containing silicon, which is formed below the photoresist layer 100.
Further, when an opening is formed in the hard mask layer 103 by performing etching using a plasma, the opening is formed to have a tapered cross section by depositing a deposit generated from the plasma in the opening. If the plasma is generated by using C4F8 gas or C4F6 gas, the plasma effectively etches the hard mask, and generates a large amount of CF-based reaction products that can be used as deposits by reaction with silicon in the hard mask layer. That is, C4F8 gas or C4F6 gas is widely used in order to improve the etching efficiency and facilitate the formation of a taper shaped opening.
However, a large amount of CF-based reaction products thus generated are adhered not only onto the opening but also onto surfaces of components in a chamber for processing a wafer. The adhered CF-based reaction products are decomposed and generate fluorine during following etching of, e.g., a lower photoresist layer formed below the hard mask layer. A plasma generated from the fluorine affects etching of the lower photoresist layer, so that the opening formed in the lower photoresist layer may not be formed in a desired shape.
Moreover, when a width of the opening formed by etching the lower photoresist layer is reduced by using the plasma generated from C4F6 gas, a large amount of CF-based reaction products is generated and deposited not only on the opening but also on the remaining hard mask layer. The CF-based reaction products deposited on the hard mask serve as a mask of the hard mask layer during following removal of the hard mask layer or the lower photoresist layer, so that it becomes difficult to remove the hard mask layer or the like.